Technical Field
The present invention relates generally to load control devices, and more specifically to a lighting dimmer that provides sinking and sourcing current outputs.
Background Art
Traditionally, 0-10V dimmers were used for providing dimming to fluorescent ballasts. These 0-10V dimmers have recently been adapted for dimming light emitting diode (LED) drivers. 0-10V dimmers provide a direct current (DC) voltage control signal that varies between zero and ten volts. The LED driver and fluorescent power supplies determine the desired light level based on the voltage level provided by the dimmer. However, as LED technology became more prevalent, many LED drivers have come to market that do not source current, but require a dimmer capable of sourcing current on its control port.
Classical 0-10V dimmers provide a sink only output, i.e. they can only sink current. The ballast or driver is the current source. FIG. 1 illustrates a traditional 0-10V dimmer circuit 100 that provides sink only output to a current sourcing driver 101. The sink only dimmer circuit 100 utilizes an operational amplifier (opamp) 103 connected at its inverting input (−) to a control voltage input Vin 102, at its non-inverting input (+) to a feedback loop 106, and at its output to the base (B) of a negative-positive-negative (NPN) type transistor 104. The NPN transistor 104 is connected at its collector (C) to the output voltage signal Vout 108 that is used to control the current sourcing driver 101. The NPN transistor 104 is connected at its emitter (E) to the ground 109. The current sourcing driver 101 typically comprises a direct current (DC) power supply 111 that provides a voltage source to a load 110. The load 101 couples to the collector (C) of the NPN transistor 104, which provides a grounded connection to the load 110. The NPN transistor 104 provides a current sink when the sink only dimmer circuit 100 is connected to the current sourcing driver 101—i.e., current flows from the driver 101 through the NPN transistor 104 to the ground 109. However, the sink only dimmer circuit 100 cannot operate with current sinking drivers.
FIGS. 2A-2B illustrate a standard push-pull drive circuit 200 with an opamp feedback loop, which may be adapted to be used in a dimmer to sink or source its output. Such a circuit 200 comprises an opamp 205 connected at its non-inverting input (+) to a control voltage input Vin 203, at its inverting input (−) to a feedback loop 206, and at its output to the bases (B) of a NPN type transistor 208 and positive-negative-positive (PNP) type transistor 209. The NPN transistor 208 is connected at its collector (C) to a positive power supply Vpos 210 and at its emitter (E) to the output voltage signal Vout 212 and to the emitter (E) of the PNP transistor 209. The collector (C) of the PNP transistor 209 is connected to a negative power supply Vneg 211.
Depending on the type of driver a dimmer containing circuit 200 is connected to, the feedback loop 206 will turn either transistor 208 or transistor 209 on. For example, as shown in FIG. 2A, the output voltage signal Vout 212 of circuit 200 may be connected to a sinking driver 201, which typically comprises a load 215 connected to a ground 216. When circuit 200 is connected to a non-sourcing or sinking driver 201, the 0-10V control voltage input Vin 203 causes current to flow from Vpos 210 through the NPN transistor 208 and to the output load 215 of the driver 201. Accordingly, the load 215 is pulled up to receive voltage from the positive power supply Vpos 210. The opamp 205 and feedback 206 ensure the output voltage Vout 212 matches the input signal voltage Vin 203. The positive power supply Vpos 210 is required to be greater than 10V to allow for the VBE drop of transistor 208 and provide headroom for the opamp 205. For example, a suitable value for Vpos 210 may be 14V.
As shown in FIG. 2B, when a dimmer containing circuit 200 is connected to a conventional driver 101 containing a load 110 that is sourcing current, current flows from the load 110, through transistor 209 and to the negative power supply Vneg 211. Circuit 200, however, requires the addition of the negative power supply Vneg 211 for generating a negative voltage. A negative power supply Vneg 211 is required to compensate for the VBE drop of transistor 209 and provide headroom for opamp 205. Inclusion of such a power supply 211 in a dimmer circuit generates a significant cost and space burden. Additionally, circuit 200 requires a PNP type transistor 209, which is generally more expansive and less readily available than a NPN type transistor.
Accordingly, there is now a need for improved lighting dimmer that can provide sinking and sourcing current outputs.